Flexible, expanded materials comprising a polymer (blend) for thermal and acoustic insulations are well established in the market since decades. They are used for commercial and residential construction as well as for industrial applications in various industries. The reasons for choosing such materials are manifold: They are watertight and therefore prevent corrosion under insulation, have excellent thermal and acoustic insulation properties and they are easy to apply due to their flexibility, easy cuttability and bondability with one-component adhesives.
The polymeric insulation foams used for such applications comprise mainly two groups of materials, polyethylene foams (PEFs) and flexible elastomeric foams (FEFs).
Polyethylene foams (PEFs) are made by a physical expansion (foaming) process, using physical blowing agents. Flexible elastomeric foams (FEFs) are flexible insulation materials with a high filler loading, achieved by a chemical expansion (foaming) process. Such materials are almost exclusively based on a narrow selection of polymer (elastomer) bases. The majority of such expanded materials are based upon acrylonitrile butadiene rubber (NBR) or NBR/polyvinyl chloride (PVC) (e.g. NH/Armaflex®, AF/Armaflex®, K-Flex® ST, Kaiflex® KKplus), ethylene propylene diene rubber (EPDM) (e.g. HT/Armaflex®, Aerocel® AC) and polychloroprene (CR) (Armaflex® Ultima). Expanded EPDM is mainly used for higher temperature insulation, e.g. solar applications, CR is used for applications where high flame resistance and low smoke emission are requested and NBR is the most widespread polymer base for standard FEFs, such as in heating and plumbing and ventilation and cooling applications. Due to the high loadings feasible with such materials, the product properties can be modified in a wide range, e.g. concerning flame resistance, thermal conductivity, mechanical properties, water vapour resistance, etc.
Since the late 70s of the last century, the expansion of FEFs is achieved by using azodicarbonamide (ADCA) as a chemical blowing agent. ADCA is one of the most effective blowing agents and therefore widely used, not only for FEFs, but for cellular rubber and thermoplastic in general. It has the highest gas yield of all commercially available blowing agents (≈220 ml/g) and decomposes mainly to nitrogen and carbon dioxide. Decomposition of pure ADCA starts above 200° C. (≈220° C.), but such temperature can be significantly decreased in a wide range by using—among many others—zinc containing substances, especially ZBS (zinc benzenesulfinate dehydrate) and ZnO (zinc oxide).
In December 2012, ADCA was added by the ECHA (European Chemicals Agency) to the candidate list of substances of very high concern for authorisation due to the equivalent level of concern having probable serious effects to human health. Although there is currently no clear evidence for such effects, there is still a risk that the use of ADCA will be limited or restricted. Therefore, there is a need for substitutes of the same or comparable performance. Unfortunately, the substitution of ADCA within FEFs is particularly difficult, as the densities of such foams are very low (at least below 70 kg/m3, for the majority of applications below 60 kg/m3 or even below 55 kg/m3). Such densities are unavoidable to achieve the required properties, e.g. low thermal conductivity, flexibility, bendability, etc.
Chemical blowing agents can in general be divided into two major groups: endothermic and exothermic materials. Exothermic blowing agents liberate a higher amount of gas compared to endothermic blowing agents and create a higher gas pressure. The decomposition temperature of such products can in many cases be adjusted (means lowered) by addition of kickers. Endothermic blowing agents are based on inorganic carbonates or hydrogen carbonates and release mainly CO2 (carbon dioxide) and in many cases water, too. They can be activated by acids, e.g. citric acid, to reduce their decomposition temperatures.
The amount of commercially available exothermic blowing agents is very limited. Apart from ADCA, only five additional substances are of commercial interest:                1. OBSH (4,4′-Oxobisbenzene-solfonylhydrazide),        2. TSH (p-Toluenesulfony-hydrazide),        3. TSS (p-Toluolsulfonyl-semicarbazide),        4. 5PT (5-Phenyl-1H-tetrazole) and        5. DNPT (N,N′-Dinitrosopentamethylene-tetramine).        
Concerning gas yield (190 ml/g) and decomposition temperature (≈200° C.), DNPT is the most comparable one to ADCA of all aforementioned chemical blowing agents. Unfortunately, it releases nitrous gases through decomposition and would therefore not be alternative, especially in respect to human health effects.
The decomposition temperature of 5PT (≈240° C.) is even higher than ADCA decomposition temperature and no kicker is known for such blowing agent. Due to this, 5PT would also not be an alternative to ADCA, as FEFs cannot be processed at such high temperatures (degradation of e.g. polymer chains, cross-links, flame retardants, etc.).
Decomposition temperature of TSS is ≈220° C., but kickers are available (urea, PTA and NEt3). However, TSS is suspected to be carcinogenic in bioassays and semicarbazides are in general in focus of WHO. Due to this, even TSS is not an acceptable alternative to ADCA for FEFs.
TSH has the lowest decomposition temperature of the exothermic blowing agents (≈145° C.), but the decomposition temperature is further reduced to 100-130° C. when used within FEF compounds. Due to such decomposition temperature, no sufficient crosslinking can be achieved prior to expansion, resulting in significantly higher densities and instable and/or open cells. Furthermore, the gas yield of such blowing agent (≈100 ml/g) is quite low compared to ADCA.
The only remaining blowing agent is OBSH. Although the decomposition temperature (≈160° C.) and gas yield (≈125 ml/g) are significantly lower compared to ADCA, it is feasible to produce FEFs using such blowing agents. However, significant adjustments of cure package and process conditions are necessary to achieve sufficient densities and product qualities. Nevertheless, there are several drawbacks of using OBSH: The resulting products are firmer (less flexible), elasticity and recovery behavior are worse. They are typically of higher density (US20100065173) and therefore have significantly higher thermal conductivities; or they are not closed cell foams (CN104945746 and U.S. Pat. No. 8,353,130) and therefore have worse water vapor barrier properties (WVT according to EN 13469/EN 12086:<1000).
A large number of endothermic chemical blowing agents is commercially available, although the amount of different raw materials is quite limited. The major reason for such a huge amount of commercially available products is the broad variety of mixtures, ratios, particle sizes, activation etc. of such raw materials, individually composed for the target applications. Although such blowing agents are preferred regarding health and environmental risks, the required densities cannot be reached (lowest achievable densities are above 200 kg/m3).